Hydrocarbon streams such as natural gas often include carbon dioxide in large concentrations, especially when extracted from a well. Carbon dioxide content of the hydrocarbon streams can reach concentrations of about 50 mole % or more. Carbon dioxide is corrosive and non-combustible, so it is not desired in the hydrocarbon streams. Some natural gas pipelines establish a maximum carbon dioxide concentration of about 2 mole % or less. Hydrocarbon streams used for liquefaction frequently have a carbon dioxide concentration limit of about 50 parts per million by mole (ppm) or less, because higher concentrations will form dry ice deposits as the hydrocarbon streams are liquefied. Carbon dioxide is frequently removed from hydrocarbon streams with an aqueous amine solvent, where the carbon dioxide reacts with the amine but not with the hydrocarbons in the hydrocarbon stream. Typically, the hydrocarbon stream is passed upwards through a packed bed while the amine solvent flows downward. The amine solvent is then regenerated and re-used. A downstream adsorption unit is commonly used to remove water from the hydrocarbon stream after removal of carbon dioxide.
During separation of the carbon dioxide from the hydrocarbon stream using the amine solvent, the amine solvent must pass through the packed bed at a sufficient flow rate to absorb the carbon dioxide, and the packed bed, the pumps, and the regenerator are sized for the amount of carbon dioxide to be removed. Under circumstances where the hydrocarbon streams include low amounts of carbon dioxide, such as from greater than about 50 ppm to about 3 mole %, an amine solvent circulation rate results in a very low liquid loading of the packed bed and may result in inadequate wetting of the packed bed. Further, off-shore facilities that employ the amine solvent to remove carbon dioxide will rock and move with wave and wind action, and the motion temporarily tilts the packed bed to thereby further impact sufficient wetting of the packed bed. The efficiency of the packed bed is reduced when tilted because the amine solvent accumulates on the lower side of the packed bed while the hydrocarbon stream moves more rapidly through the upper side of the packed bed due to the reduced flow resistance from the decreased amine solvent flow. On many off-shore facilities, the packed bed, amine solvent pumps, and related equipment are oversized to account for the motion of the facility. The increased sizes of the packed bed and pumps increases the capital expense to build and install the packed bed, and also increases the operating expense to recirculate the amine solvent.
Alternative separation schemes have been proposed for separating carbon dioxide from the hydrocarbon streams to avoid the aforementioned challenges associated with packed beds. Membrane elements have been used to absorb carbon dioxide from natural gas and other vapor streams. The membrane elements do not use a packed bed, so the motion of an off-shore facility does not significantly change the operating efficiency and the membrane elements do not have to be oversized for the desired service. However, use of membrane elements results in significant loss of hydrocarbons and is inefficient when the hydrocarbon stream includes the relatively low amount of carbon dioxide. Adsorption schemes have also been proposed for separating carbon dioxide from hydrocarbon streams. However, a raffinate stream that results from desorption of the carbon dioxide is not recovered and may result in a waste stream that requires further remediation.
Accordingly, it is desirable to develop methods and apparatuses for removing impurities such as carbon dioxide from hydrocarbon streams while avoiding challenges associated with inadequate wetting of packed beds that use an amine solvent to remove carbon dioxide from hydrocarbon streams that have a low carbon dioxide content. It is also desirable to develop methods and apparatuses for removing impurities such as carbon dioxide from hydrocarbon streams that maximize hydrocarbon yield from the hydrocarbon streams. Furthermore, other desirable features and characteristics of the present embodiment will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.